Water-borne paints, primers, coatings, inks, adhesives, and other water-borne film-forming compositions (“water-borne compositions”) may comprise solvents, binder, pigments and other additional components that favorably affect various properties of the compositions, such as polymeric surfactants and dispersants. The binder and the pigment, if present, are the primary solid components of the water-borne composition and remain on surfaces to which they have been applied forming a solid layer as solvent evaporates. In water-borne compositions, the primary solvent is water. Typical water-borne compositions include latex paints.
Polymer latexes or dispersions are widely used in water-based coatings. The polymer latex or dispersion is dried to form a film via coalescence to obtain desired mechanical and physical properties. Coalescence is a process whereby polymer particles in aqueous dispersion come into contact with one another during drying, and polymer chains form (i.e., diffuse) across boundaries of latex/dispersion particles to yield continuous films with good bonding of the particles.
A method of improving the properties of films formed by water-borne compositions is to include polymers that are capable of cross-linking. The polymers may be self cross-linking or dependent on the involvement of a cross-linking agent to react with the polymers.
Of the aforementioned polymers, those that are self-cross-linking containing components such as acetoacetoxyethylmethacrylate (“AAEM”), glycidal methacrylate, and oxidative or UV curable functional groups are typically used in (among other things) water-borne coatings, inks, and adhesives. Unfortunately, self-cross-linked polymers yield only minimally improved mechanical properties in the resultant films, particularly for scrubability of water-borne paints. On the other hand, cross-linked polymers formed with the involvement of a cross-linking agent have the potential for exhibiting superior performance.
That is to say, cross-linking technology based on the use of a cross-linking agent to cross-link binders and/or other polymeric additives such as polymeric surfactants and dispersants (U.S. Pat. No. 5,348,997) or rheology modifiers (U.S. Published Patent Application No. 2009/0162669) is comparatively attractive because it affords the possibility of realizing one or more property improvements of a substantial and commercially significant nature. To with, cross-linking of polymers in coatings, inks, and other water-borne film-forming compositions can improve physical and mechanical strength, adhesion, and durability. In coating applications, cross-linking may also improve scrubability, blocking resistance, chemical resistance, and weatherability (see U.S. Pat. No. 7,547,740 and U.S. Patent Application Publication No. 2007/0265391). Cross-linking of polymer constituents may also enhance adhesion and bonding strength for adhesives. Additionally, cross-linking is widely utilized in the formulation of printing inks, in order to improve the mechanical and chemical resistance of prints.
Accordingly, it is to be understood that cross-linking during and after coalescence with a cross-linking agent further enhances the physical and mechanical properties of the films. However, if the polymer cross-linking reaction begins before the water-borne composition is applied to a surface, the resultant film may have poor physical and mechanical properties. Premature cross-linking is that which occurs within the polymer latex or dispersion, such as a latex or dispersion of a polymer which has carbonyl and/or epoxy functionality, before it is applied to a surface. It may prevent coatings from forming coalesced films.
Premature cross-linking in compositions used for paints, inks, adhesives, dispersants, polymeric surfactants or rheology modifiers can also be disadvantageous in causing viscosity instability, thus reducing shelf-life. For example, typical latex base paints are manufactured in bulk and then dispensed into retail containers (such as a can) for shipping and storage. The retail container and its contents may be kept in a warehouse or a retail store prior to sale and ultimate use. During this storage period, cross-linking of the polymers may occur, thereby reducing the quality of the latex paint. The cross-linked polymers may gel and/or build up viscosity in the can and a paint film formed therefrom can have reduced physical and mechanical properties such as reduced scrubability.
For instance, in an effort to improve properties, water-borne compositions have been produced with polymers comprising a monomer such as diacetone acrylamide (“DAAM”) and the cross-linking agent adipic acid dihydrazide (“ADH”). Without significant pre-application cross-linking, such water-borne compositions give improved properties over paint comprising non-cross-linkable polymer, but the improvement is not realized if there is any substantial pre-application cross-linking. Studies have shown that the cross-linking of polymers comprising DAAM with ADH cross-linking agent through a keto-hydrazide reaction has a substantial reaction rate in an aqueous solution. (“The diacetone acrylamide cross-linking reaction and its influence on the film formation of an acrylic latex”, Journal of Coatings Technology and Research, 5(3), 285-297, 2008.) Consequently, premature cross-linking impedes the attainment of desired properties.
An attempt to address the problem of cross-linking prior to application of a polymeric composition containing a polyhydrazide or the like appears in U.S. Pat. No. 4,786,676. There it is taught that polymeric compositions which have a long shelf life in aqueous solutions or dispersions and which undergo cross-linking after drying at room temperature or elevated temperatures are obtainable by reacting polymeric organic compounds possessing carbonyl and carboxyl groups with polyhydrazides in the presence of monoketones and/or monoaldehydes. However, the development discussed in the patent is not adequate for reliably suppressing premature cross-linking because there are side reactions which introduce impurities that degrade the desired stoichiometry, because any chemical species that might prevent premature cross-linking would not be produced in sufficient quantity within a time frame that the undesired reaction could be precluded, and because the use of (at least) ketones can present a safety hazard. It can be seen that the patent's teaching does not afford a practical solution.
Thus, there exists a need for a water-borne composition, comprising a polymer having carbonyl and/or epoxy functionality, with improved physical, chemical and mechanical properties, such as, scrubability. Further, there is a need for a water-borne composition wherein a component cross-linkable polymer does not undergo significant cross-linking prior to application of the composition, especially for water-borne paints. But that is not the full extent of the conventional technology's shortcomings. More specifically, various polymers comprising vinyl-containing monomer units that upon hydrolysis yield acetaldehyde (e.g., vinyl alkyl ester units, especially vinyl acetate units), such as vinyl acetate-acrylic and vinyl acetate-ethylene copolymers, are recognized in the art as being highly advantageous when utilized as the binder in an aqueous latex paint or other coating. However, in our experience it can be even more difficult to elicit satisfactory performance from these specific polymers than previously described. The question arises whether, with such vinyl-containing polymers, there is yet another undesirable phenomenon which must be identified and counteracted in order to realize the substantial performance potential from cross-linked binder materials.